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Could a terrorist hack the ILS?
I'm reading all these reasons why consumer electronics can't be on during takeoffs and landings and one specific reference that is made over and over again is the sensitivity of the ILS equipment. I understand that isn't a Die Hard 2 scenario, but what about simply throwing random interference on those frequencies hoping to disrupt navigation? I know very little about RF or the power levels involved, but a generator inside a hotel room in or around the airport property sounds like a feasible scenario.

Compare the sensibility of the ILS with the sensibility of the GPS: Same transmitting power (about 25W), ILS at 20 km, GPS at 20,000 km at best, when at zenith. ILS is really not difficult to receive.

+1 for the Die Hard 2 reference, because that's definitely the first thing I thought of when I saw the subject line.

@mins, comparing ILS (really LOC) with GPS is an unfair playing field.  Consider that most GPS receivers in existence use autocorrelation methods to extract usable signal when it is almost at the noise floor. I know of no LOC or even GS receiver designed for that. Of course the beauty of GPS is that you can have hyperbolic nav with many transmitters all at once. On top of that there are pseudorandom sequences in GPS which in practice make it just a little more difficult to spoof. But don't be fooled into thinking that GPS is secure, as it is rather insecure.

@mongo: In response to "one specific reference that is made over and over again is the sensitivity of the ILS equipment", I mention ILS signal available at the aircraft location is very much larger than e.g. available GPS signal, so ILS receivers don't have to be, and are not, as sensitive as the GPS one. I believe the comparison is useful. I don't say GPS is secure or not.

There are numerous unclassified papers talking about LOC, GS, MLS and other navaid jamming. There are also instances of that happening, both intentionally and unintentionally.  The issue with consumer electronics is made worse is that local consumer electronics on the plane can have local oscillators in the receivers.  They are small signals, but they are close to the antenna, hence the greater risk.
Jamming LOC (lateral ILS guidance) and VOR are similar. GS interference is a bit more difficult to provide false readings that make sense to a pilot, when the transmitter is off airport.
The reality is that even GPS can be jammed, faked with stronger signals, etc. Numerous companies make GPS-GNSS simulators, and it is not rocket science to integrate those into jamming systems.  Some of the countermeasures are documented in open literature, and other countermeasures and methods are not.  GPS jamming becomes an issue with various weapons delivery systems, and creating effective countermeasure packages with limited size-weight and power (SWAP) is a constant challenge.
I concur with @STWilson, that if faced with the possibility of interference, it would be good practice to monitor your approach with GPS, NDB, OM and other navaids.  For example, one can always start a timer crossing the OM, even if doing an ILS. Certainly in a war or terrorism situation, additional cross-checks are good investments of effort.

Yes, An ILS could be jammed or disrupted.  It is a narrow directional signal emitting from an antenna at the runway.  A pilot uses other means of navigation to intercept its signal and verify the legitimacy of its course.
A pilot is trained to cross-check position and to notice irregularities of the ILS indication.  She or he is taught to be suspicious of erratic and bogus ILS indications, though this isn't taught in the context of possible sabotage.
Fortunately today, most modern aircraft have GPS, the most robust secondary nav source for an ILS approach, compared to VORs, NDB/LOMs and marker beacons.  Pilots will often perform an overlay approach, in lieu of older kinds of approaches, like the ILS .  These are so-called because most GPS approaches fly exactly the same path as if the GPS chart is "overlaid" on an older approach chart's path.  There are GPS options for most all older approaches.  Nevertheless ILS approaches in their original form remain in common use as default option offered by ATC.
Precision (meaning with glidepath guidance) WAAS GPS approaches are becoming the norm with pilots over ILS and other options. An LPV approach is one such example.

Not to mention that most landings are done by the Mk 1 Eyeball, at least as a backup.  So someone intent on causing a crash through interference with ILS signals would have to hang around waiting for instrument conditions.


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What plane is shown here in the movie Rampage?
I'm not sure if its a real life airplane, but I feel like I have seen it somewhere:

It looks similiar to the F-117 Nighthawk, pictured below, but it is definitely not this:

That's a pretty stupid way of using a stealth bomber, ouch.

@JörgWMittag Funny thing is they'd probably use a B-52 for an operation like that, but few would recognize it, and they fly too high for dramatic shots like this. I noted the movie Cloverfield also used a B-2 for dramatic effect

@Machavity Granted, the B-2 with its 50,000 ft service ceiling doesn't exactly fly low, either.

@JörgWMittag, why would you say so? In the movie they are attempting to drop bombs on 3 very large animals and want to avoid being seen?

@KSplitX: Because the purpose of the B-2 is stealth, and they are flying around low and slow in broad daylight. If they want to avoid being seen, they are doing almost everything wrong that they could possibly do wrong. I mean, the guy in the third screenshot is clearly looking directly at the bomber: not being seen 100% FAIL. The underside of the plane is painted black to be hard to see against the night sky, it is easy to spot against the blue day sky. It is designed to fly very high outside of normal visual range.

@JörgWMittag it's actually black (or rather a very dark grey) mostly because that's the colour they ended up with when designing the radar absorbent paint. They just didn't bother adding white pigment to turn it air force grey like they did with the F-22 and now F-35. That it makes the B-2 harder to see at night is a bonus, not the original intent.

Northrup Grumman B2 Spirit stealth bomber.

This answer would be improved by showing the B2 from similar angles to the pictures above

@Richard Did you look at the OP's third picture?

I did, but they've shown it from various angles.

source: Stack Exchange

license: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/

sort: votes


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What is this hole on the left wing of the Eurofighter Typhoon?
I came across the picture below and saw the hole near the centerline on the left which appears to have some carbon next to it. What is its purpose and where does the carbon come from?

Cartridge starter?

The Auxiliary Power Unit (APU) exhaust

(Source)
The item 72 is what you are looking for and it is labelled "APU exhaust". The black residue is soot from incompletely burned fuel.
The Eurofighter Typhoon can use its APU to start the main engine or to provide power when on the ground. From airvectors.net:

The Eurofighter is also fitted with an auxiliary power unit (APU) for self-starting and ground power.

See also:

What is the APU actually used for?
Do fighter jets have an auxiliary power unit?

But what do  Kwik-E-Marts have to do with jet fighters?

... if that is, in fact, your name ... -- Chief Wiggum

why is it placed there like that instead of a backwards pointing nozzle somewhere else?

@user1062760 You can see the APU itself (item 71) right below, so it would make sense to have the exhaust right next to it to save on additional tubes (and weight). I don't know why the APU is placed there, but I assume that was just a place where it could fit.

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/4.0/


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How does the Coanda effect help some aircraft produce more lift?
How exactly does the Coanda effect affect the air stream on the wing of, e.g., an AN-72?
The Coanda effect makes an air stream flow on the curved (outboard) surface object which it has produced from there. How does it help produce lift?

The Coanda Effect only explains why the airflow doesn't separate as it encounters the leading edge; it's not what generates lift.

But if you see this: http://yon.ir/Y60Jc you will understand that AN-72 uses Coanda effect to produce lift in low airspeed condition.

It's not so much that the Coanda effect is used to produce lift, but rather that the engines (and similar devices) enhance the Coanda effect, encouraging the airflow to stick to the wings, thus not separating, especially at high angles of attack, which would occur at low airspeed.

According to Wikipedia, the Coandă effect causes the airflow over the top of the wing to "stick" to the upper surface of the wing and continue to hug the wing surface even as said surface curves downwards.  In normal flight, the airflow remains attached to the wing surface all the way to the trailing edge of the wing; as the upper surface of the wing's trailing edge (and, for that matter, generally the lower surface as well) is angled downward, the airflow over the wing is travelling downwards when it leaves the trailing edge, which, as per Newton's third law, produces an upwards force on the wing (and, thus, the aircraft), which we know as lift.  The An-72's overwing engine placement increases lift at low airspeeds because it causes the engine exhausts to blow over the top of the wing; the high-speed exhaust sticks to the upper surface of the wing thanx to the Coandă effect, and is thereby ultimately deflected downwards by the downwards curvature of the rear portion wing's upper surface, producing good lift even at low airspeeds.
A simple way to think about it would be that the An-72's engines blow air and hot gas downwards as well as backwards, but, instead of doing so via traditional ducting, they blow air downwards by blowing it over the top of the wing and allowing it to stick to the upper surface of the wing and follow said surface down.
Note that this is not the only, or even necessarily the primary, mechanism by which aircraft generate lift; it just happens to be the mechanism that is enhanced by the An-72's engine placement.


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Why is the flight duration of the same flight different in different months?
For example:

CX880 (HKG TO LAX) takes 13h40m on the 2nd of August, but only takes 12h50m in September.
CI008 (TPE TO LAX) is the same; it takes half an hour less in September than in August.

What are the factors that cause this difference?

Are you asking about scheduled flight times or actual flight times?

https://aviation.stackexchange.com/q/31576/3573

@J... If you think it's a duplicate, you should flag it as such. Duplicate closure is not a bad thing.

@Machavity Yeah, I would, but I left all my rep at SO.

@J... You have more than 15 rep. You can flag a question for closure, even if you can't vote for it directly

@Machavity Hah...right, good point.  I've had CV priv at SO for so long I forgot you can do that.  Anyway, I hate cluttering flag queues and it's HNQ so I figured it would get dealt with soon enough in any case.

Jet stream is a stream of air flowing around the Earth from West to East at high altitude. With wind speed averaging more than 100+ km/h, it has a tremendous effect on a plane's ground speed.
The pattern, strength and location of the stream is constantly changing. For example, during the (Northern Hemisphere) winter the stream generally strengthens, and during the summer it generally weakens. With modern technology such as weather satellites and data fusion from thousands of weather stations around the world, the stream is fairly predictable.
The effect is more pronounced on long flights because these flights stay in high altitude much longer.


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How do internally carried IR missiles acquire a lock?
On aircraft that carry their IR missiles on external stores, like the F/A-18, an IR-missile acquires a lock by having its seeker being directed to the target heat source by whatever means and the missile then locking on the the heat contrast with its own seeker. The missile only gets fired once the seeker has a positive lock.
How would this work in an aircraft that carries its IR-missiles internally?
Would the weapons bay doors have to be open to let the missile acquire a target with its own seeker or would the missile just be given an approximate position where to search and it would then acquire a lock on its own once it's out of the weapons bay?

"The missile only gets fired once the seeker has a positive lock" - I don't think that is a valid assumption.

As far as 4-gen fighters and before go I would say its correct.

If you are referring to the Sidewinder missile, the first iteration AIM-9X did not have a requirement to be launched from internal stores, known as captive carry, but in testing it demonstrated that it could lock on after launch. This was deemed interesting enough that Raytheon got a contract for the Block II variant, which among other things adds a datalink capability so that an aircraft with the right equipment can direct the missile to where the target is located, after which it locks on with the IR sensor. This allows it to be used on the F-35, F-22 and even submarine platforms.$^1$
Block III has the same datalink capability (derived from tech developed for the AMRAAM missile) but also has increased range and an insensitive warhead that reduces possibility of accidental detonation on the ground, among other things.
A big reason why the AIM-9 series continues to be upgraded is because of its IR capability. Some countries have developed jamming technology that can interfere with the more modern AIM-120's radar tracking system.

$^1$: Raytheon AIM-9X Block II Missile Completes First Captive Carry Flight, spacewar.com, 2008

And AFAIK the launchers are moved into the airstream prior to launch, giving the missile time to lock on (though you'd have a several second delay of course, not ideal).


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How do I improve navigation with a VFR sectional chart?
I am doing my PPL atm. I am struggling to navigate with a VFR sectional chart, I am getting lost more often. How do I fix this issue? Any tips?

I have trouble identifying small towns, identifying places if there are no big landmarks.
How do I identify places more accurately?

If your question is "how to identify places with only a sectional chart on board?" the title should reflect it. As is, the title don't tell anything about identifying places. Moreover, as we are on a Q&A website, you should formulate the title as a question.

The trick is not to fixate on small details, but to scan on the chart along your route for landmarks and features that combine into large geometric shapes that are easy to pick out from the air (while cruising the higher the better).
Like; highway, river, and railway line that form some shape like a large 5 mile long triangle so that there is a unique pattern, with a town at some point on the triangle that makes it especially distinctive, to the left of your track.
Note that configuration on the map.  Try to mentally compress it in your mind to allow for foreshortening because you will be initially viewing it at an angle.  Look for its match on the ground, allowing for its location to be farther left or right than expected if you've drifted off track.  As you find it and know where you are, scan ahead on the map for another easy to spot shape, as close as possible to your current position, note its distance, estimate the time to get there based on your ground speed and wait for it to appear.
Beyond that, it's mostly about establishing the correct heading to stay on track and holding it well, staying on top of your position and not allowing distractions to divert your attention too long as the world slides by below, and getting good at mentally projecting your progress based on known ground speed (like, you've been distracted for 5 minutes since your last known position, and if your ground speed is 2 miles per minute you should be between 9 and 11 miles away from it, so start looking for a recognizable geometric pattern ahead of you, say, 12 to 15 miles from last known).
A home computer flight sim like FSX or Xplane is really great for practicing this sort of thing on your own time.

The way I was taught was not intuitive at first.
After letting me try my hand at flight planning on my own, the first thing my instructor told me to do was to stop trying to fly from visual reference point (VRP) to visual reference point. That is not the purpose of the landmarks.
Instead, plan a route that is as direct as possible. There are going to be times that you have to deviate from a true straight line course. Try to limit the number of changes in heading as much as possible.
Then, pick Your VRPs based on what would be visible from your route. For really large landmarks, that would limit the lateral distance from your route To a maximum 10 miles. You don’t have to fly directly over these VRPs. Flying abeam them is sufficient. Try to have you VRPs alternate each side of your airplane. That way they can bracket your course. Try to make your VRPs no more than 10 minutes apart. And, only make course direction changes at major, easily identifiable, VRPs.
Do NOT pick landmarks that can be easily mistaken for other landmarks. NOR landmarks that are so big that they become ambiguous. Cities are too big to use unless you use a point in the city. Small bodies of water can be misidentified unless combined with another VRP. Large bodies of water are too large to keep you on the right track. Only major highways can be used to fly roads. Power lines and other towers are harder to see at altitude than you think. Streams and small rivers can look completely different at different times.
Here is a list of good VRPs:
Airports with hard surfaces;
Major highway intersections;
Large bridges;
Dams;
Uniquely shaped bodies of water;
Familiar shopping malls;
Well marked sporting venues (when not in use);
Antennae Farms;
Wind turbine Fields;
Large groups of oil tanks;
MAJOR AND FAMILIAR geographic features;
Large industrial complexes like refineries & power plants;
Items on your sectional that are marked Visual Reference Points.
Use technology to assist you. Someone has already mentioned Google Earth. Review the satellite image of each of your VRPs in order and at the point of view and altitude of your flight. Also, ForeFlight has a 3D satellite review of your flight plan that you can watch at realistic speed or up to 20 times actual speed.
Cross reference your VRPs using VORs.
Don’t be afraid to fly from VOR to VOR if you are on Flight Following. As a matter of fact, don’t be afraid to fly Victor Airways if you are on Flight Following. And, by the way, USE FLIGHT FOLLOWING. It’s hard to stay lost when someone always knows where you are.
Remember the 10x10 rule. Your VRPs should be no more than 10 miles off of your course and no more than 10 minutes between each other.
File, open, and close a flight plan.
I hope this helps.
CAVU and smooth air.

John K pretty much totally nailed in his answer, as a sort of a side note I'd like to add that it helps to understand how we perceive things:
We do not see everything around us: if we did, our brains would explode in less than a microsecond because of information overload. Our visual cortex is only triggered by stuff our deeper "processes" regard as obstacles or threats, and by things we are looking for.
Then category of "things we are looking for" must be correctly tuned to work. This is what Jonh K well addressed in his answer. If you are looking for forms and shapes as they appear in the map, your eyes, or actually your brain will not be able to catch them in the landscape around you because they do not manifest as on the map. You first have to translate the map in your mind to the perspective you see from your seat, only then will you have any chance of seeing things you are looking for.
I would not worry, this is something that can totally be learned, the difficulty is that the best way to learn it is by doing it, and the only thing that comes close is, as John K suggested, is using a simulator. Unfortunately simulators have the drawback of delivering the landscape to us in 2d format (flat screen), so it is a bit different from the real thing, but definitely helps in teaching your head the fine art of translating the 2d map to 3d expectation to look for.
If you think you can see things you are not looking for, try this classic experiment yourself: The selective attention test by Simons & Chabris
P.S. I presume you are holding the map aligned to your flight path, and roughly following your position on the map with your thumb? There's no need to make things more difficult than they are :)

My advice as a CFI is to pick waypoints that:
1) Readily identifiable and distinct when viewed from all directions.  Many people select waypoints like small airports, country roads, etc without really being awareness of how easy these are going to be to spot from the air.  Good waypoints are natural things like oxbow lakes or bends along a river, tips of finger lakes, small buttes, small lakes or ponds.  Many man made features are excellent references like power plants, freeway interchanges, concrete plants, substations, sport stadiums, pit mines, small isolated towns in the countryside, prisons, large bridges, water towers, dams, etc.  Major airports may be used but I’d refrain from using small, country airports as they are often very difficult to pick out from the countryside if you are unfamiliar with the area.  In addition, I like to review satellite photos using Google Earth, etc over the proposed route and see if all my selected points are readily identifiable.
2) For whatever visual waypoint you choose, make sure there are other visual references nearby which you can orient yourself to to locate your position relative to your chosen waypoint.  For example, you’re flying towards a bridge which is your chosen waypoint and you spot a finger lake a mile or so directly to the left of the bridge.  You can use this to determine when you have crossed your chosen waypoint by noting when the finger lake is directly off your wingtip and you were tracking directly toward the bridge.
The 3D trip simulator in ForeFlight is an excellent way to do a practice fly through of a proposed CC route and make sure you can easily spot all your waypoints and visual references.
3) If you’re flying an airplane with conventional instruments and a free gyro heading indicator BE SURE TO A) ALIGN THE HEADING INDICATOR TO YOUR MAG COMPASS BEFORE BEGINNING YOUR TAKEOFF ROLL AND B) PERIODICALLY RE-ALIGN THE HEADING INDICATOR WITH THE MAG COMPASS EVERY 15 MINUTES WHILE ENROUTE WITH THE AIRCRAFT IN STRAIGHT AND LEVEL FLIGHT.  You wouldn’t believe the number of solo CC students who depart on a flight and head off into the boonies because the completely forget to do this!
4) Be in a position to get a good view of your waypoints.  I personally prefer to select a VFR cruise altitude of between 4000-5000 ft AGL, obstacle clearance and aircraft performance permitting.  This provides a good ‘perch’ for navigating by pilotage, high enough that ground clutter can’t obscure your view, but not so high that it becomes difficult to spot visual details.
5) Never select visual waypoints more than 20NM apart.  This will help to both minimize wind drift errors and corrections and 2) keeps you relatively local in the event you really do get lost and require ATC assistance to locate your craft.
6) Get the latest winds aloft forecasts and re-compute your magnetic headings, preferably less than 1 hour before your scheduled departure.  This should help minimize wind drift errors.
7) Log all of your times on each enroute segment and compare these to your ETE for each segment.  Note any differences in your position relative to your waypoint and update your actual winds aloft using an E6B periodically.
8) In the movie The Patriot, Mel Gibson’s character advises his son that to shoot a gun accurately, he must aim small so as to miss small.  This is very good advice for good cross country navigation as well.  Make a deliberate effort to fly good straight and level segments with as minimal deviation from one’s desired magnetic heading as possible.  The best way to do so is once on course, pick a reference point in the distance eg a distinct farm field, a cloud, etc and just hold the nose on that point with very tiny aileron inputs.  This will minimize course tracking errors.
9) You can periodically identify your position using two VORs to get a fix.  Verify you are still on course and where you intended to be when you intended to be. NOTE: Be sure to identify each VOR with the Morse Code tone to verify that they are working properly!
10) Finally if you do actually get lost, remember to use the 6Cs - CLIMB, CIRCLE, CONSERVE, CALL, CONFESS, COMPLY to find your way back to your destination.

Once you identify a landmark on the chart, go to Google Earth (in a web browser) and look for that landmark. You can position yourself and the approximate flying altitude and attitude and see how your landmark would look like from the airplane. You can even overfly your entire route on Google Earth (it would look closer to reality that in a simulator). This method is also useful for locating obstacles mentioned in the Airport/Facility Directory around airports (antenna towers are a bit tricky to see).


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Practically, what should a pilot do to safely transit a UAS Operating NOTAM area when they must go through it?
Practically speaking, what should a pilot of a GA aircraft be doing differently when (if?) they have to fly through an area with a sUAS NOTAM?
I have flown (while in PPL training with a CFI) in a class D before with tower service and military drones. However, they were talking on airband and with a wingspan that was probably at least double that of my Cessna 150 so "see and avoid" would work, and they were in contact with the tower, and the tower kept us spaced apart.
In most cases, you could just fly around the NOTAM area. However, in the case where a drone is operating from an airport (which may be under special permission from the airport - the class D airport is apparently designated a special drone development area, and I believe the same is the case for the pictured scenario) a pilot will likely be sharing airspace with the drone.

This pictured case is at an untowered airport. What must a pilot do to operate safely in this scenario and not be in violation of 91.13? Nothing in the NOTAM states the size of the drone, so I would assume they could be smaller than a person -- and thus very hard to see and avoid.
Should the GA pilot expect to coordinate over CTAF with the sUAS pilot, or would a sUAS pilot even be allowed to make calls on the airport CTAF? Or is that disallowed, like ADSB out for drones?
If the drones had ADSB out, and talked on CTAF, I would be comfortable as I have ADSB in. If they were the size of GA planes and talked on CTAF I would be careful but consider it the same risk as flying around other GA traffic that may or may not have ADSB out.

In no way is this a complaint against the sUAS folks...I'd actually prefer flying around them as long as they are talk'n and sqwauk'n (ADSB out) over some (opinionated) NORDO GA guy that expects "see and avoid" to work 100% of the time.

Just for context, and this might end up going to chat discussion eventually, it would be interesting to know the exact location you are picturing and figure out what was the cause of the NOTAM in this case.  Maybe someone was surveying the airport with a Phantom drone, maybe a group had gotten permission to hold a model-airplane meet at the airport, etc.  In any of these cases the expectation would be that the sUA "operators" would do whatever necessary to stay out of the way of full-scale traffic-- easy to do if only operating up to 400' AGL anyway!

Talking to the sUAS operator in this case, the necessity of being at this untowered airport comes from testing and calibrating a NAVAID based on the airport. They are working on a portable CATI precision landing system that can do curved approaches with the standard LOC & GS needles and a 3A/C transponder. They say it takes them about 2 hours to go from coming in on a trailer to having a full blown ILS and PAR.

I've asked about their system before: https://aviation.stackexchange.com/q/94343/25251. I'd like to be courteous and not in their way but would also love to try flying a "bent ILS" (in VMC under VFR rules of course). I figured maybe there was a standard way the NAS separates this traffic or maybe best practice for coordination rather than the "airspace never overlaps" approach, since it looks like it could overlap with the final for runway 25. This is at the KDLS airport.

I imagine that if I were in your situation if there weren't a lot of radio traffic on CTAF I might go ahead and ask on CTAF if the unmanned aircraft/drone operators are up on on frequency and able to transmit regarding deconfliction.  Then you might be able to find out if what you want to do would inconvenience them.  I think you could be confident that they'd stay clear of you regardless especially if you state all your intentions over CTAF; on an airport project like this they are almost certainly listening.

I'm really not sure about the details of the regs regarding someone on the ground transmitting on CTAF in a case like this, and to what extent they are strictly observed or sometimes "bent".  Since the Dalles OR airport is in uncontrolled airspace all the way to the surface, my statement about the FAA possibly requiring CTAF transmissions as a condition of authorization for the sUAs operations, probably wouldn't pertain here.

This is an issue that pilots will encounter more frequently in the future than in the past.  For example, the Academy of Model Aeronautics (AMA) has negotiated a process with the FAA that allows modellers who are holding special events in Class G airspace with overlying Class E airspace to get permission to fly up the base of the overlying Class E airspace (typically 700' or 1200' AGL) rather than the generally-allowed 400' AGL, and the process involves the issuance of a NOTAM.  Also, the special areas called "FRIAs" (Federally Recognized Identification Areas) where model airplanes and other sUAs will be exempted from the new "RID" (Remote Identification)1 requirements have NOTAMs associated with them if they are near airports.
You are right that the small size of many of these sUAs (think of a typical model airplane) makes them impractical to see and avoid.  It is up to the sUA pilot to see and avoid full-scale traffic.  (See for example 49 USC 44809 (a)(4), FAR 107.19(c), and FAR 107.37 (a) and (b).)

Should the GA pilot expect to coordinate over CTAF with the sUAS
pilot, or would a sUAS pilot even be allowed to make calls on the
airport CTAF?

A GA pilot should not expect to coordinate over CTAF with the sUA pilot.  In some situations the sUA pilot may be monitoring CTAF-- the FAA has actually made this a condition of some of the special permits it has issued to allow modellers to fly as high as 2000' AGL at some locations.  But this should not be assumed to be the case generally.  As to whether it would even be legal for a sUA pilot to transmit on a CTAF frequency to enhance the safety of full-scale aircraft-- that would likely be dependent upon whether or not the FAA has specified that such transmissions be made as a condition of the authorization for the mission.2
What you should do is just operate normally.  In the vicinity of an airport, you could enhance safety by sticking to the normal traffic patterns and not doing anything too unusual.  If you happen to observe an sUA being flown in a manner that you believe to be unsafe, be sure to note your exact altitude and whether the sUA is above or below you, as well as making your best estimate of the sUAs altitude and location, and whether it appears to be a quadcopter/multicopter or a more traditional winged aircraft, so that you can submit an accurate report.
Of course, it also goes without saying that you should pay attention to the altitude ceiling listed in the NOTAM.  In the particular case in your image, it was only 400' AGL, so if you are flying circuits in the pattern at that airport, you'll be over that altitude much of the time.
Keep in mind that the FAA makes no distinction between "sUAs" (small Unmanned Aircraft or small Unmanned Aircraft Systems) and model airplanes, and model airplanes have been safely sharing the skies with full-scale airplanes for many many decades, often ranging up to 600' to 800' AGL, or much higher for some particular disciplines (e.g. soaring where 2000' AGL would not be at all unusual).  You just didn't know that they were there.  In the past the FAA has issued various recommendations that model airplanes stay below 400' AGL but these were generally not codified into actual regulations, and were often ignored by many modellers who knew that they could safely operate much higher with no appreciable risk to full-scale aircraft.  49 USC 44809, the "Special Exception for Limited Recreational Operations of Unmanned Aircraft", became law in 2018 and tightened things up in this regard, and the special permissions that the FAA sometimes now issues to deviate from this law sometimes involve the issuance of a NOTAM.  Within controlled airspace, the Special Exception allows the FAA to specify altitudes much lower than 400' as the ceiling for sUA operations, and again, special permissions to exceed these specified altitudes may involve NOTAMs.
In summary, sUAs operations for various commercial purposes are certainly becoming more common and will undoubtedly continue to do so, but another factor causing sUAs-related NOTAMs to appear with increasing frequency is the fact that the FAA has promulgated new regulations which now cause NOTAMs to be generated in relation to traditional model airplane flying at some locations where these hobby activities have already been going on unchanged for many decades. In either situation, it is the responsibility of the sUAs "operator" to avoid full-scale aircraft.
Footnotes:

In its current form, RID or "Remote ID" is a device that transmits a signal that allows law enforcement or other parties to identify a sUA in flight.  Think of it like an electronic license tag.  It does not facilitate in-flight avoidance of the sUA by other sUAs or by full-scale aircraft, though this concept was part of the original proposal NPRM.

Both FAR Part 107 and 49 USC 44809, the "Special Exception for Limited Recreational Operations of Unmanned Aircraft", require "prior authorization" for sUAs flights in controlled airspace.  Sometimes this authorization is delivered by an automated on-line system called "LAANC", or provided by standing Letters of Authorization with local ATC for operations at specific "Fixed Sites".  The FAA is free to set whatever conditions it desires, including altitude limits, as a condition of such authorization.

Just in case anyone is curious to read more, endless discussion of the new regs affecting model airplanes etc can be seen here https://www.rcgroups.com/model-aircraft-and-drone-advocacy-943/


text:
What are the meanings of those SFO Taxiway Marking?

I'm studying aviation and pulled up a Google Map satellite view of San Francisco International Airport (KSFO) and was wondering if someone could shed some light on what a few of the taxiway markings meant.

What do the straight lines starting from the taxiway edge marking towards the "median" area mean? Probably that it's not usable for aircraft taxiing - but I couldn't find out what the name of these lines were.
What is the number inside the small pink / purple used for?
What are the white numbers inside the blue boxes used for?

I find it interesting that the markings are oriented to be visible to vehicles leaving the terminal, not approaching.

For 1, what is slightly harder to see is that the line which runs parallel with the taxiway is a double line. This denotes the taxiway edge and the thicker lines running perpendicular to the double lines shows which side of the taxiway edge isnt suitable taxiway. They are often coloured differently too, and in SFO case the unusable part of the ramp is green.
The pink circles with black text are geographic position markings, and are used by the airports Surface Movement Guidance Control System (SMGCS) in low visibility.
The white numbers in blue squares are not are not mentioned in the FAA Standards for Airport Markings but I suspect thy might have something to do with the skybridges.

Thanks. I thought the yellow boxes with black numbers next to the taxiway centerlines were the gate numbers, and not the blue ones?

@jeff0000 you know what I think you might be right. Looking now for a definitive answer.

The blue boxes seem to just identify each ramp area exit. Not sure why they're distinct from the SMGCS markings.

The white numbers in blue squares are reporting positions.  When a plane leaves the gate it is to report to a blue square to hold if it does not have permission to enter onto the taxiway right away.

How do pilots know that this is the purpose if they are non-standard?  Do you know how this usage is communicated?

all_licenses: Creative Commons - Attribution Share-Alike - https://creativecommons.org/licenses/by-sa/3.0/


text:
What is the upper deck on converted freighter long-hump 747s used for?
All 747s1 have an upper deck directly behind the elevated cockpit, a legacy of the 747’s genesis as a design for a strategic-airlifter competition (the upper deck and elevated cockpit were partly to allow the use of a swing-up nose for easier cargo loading without having to disconnect and reconnect all the wiring and control cables and whatnot every time the nose was opened, and partly to keep the main-deck cargo from crushing the flightcrew in the event of an accident);2 this gives the 747 its distinctive hump and unmistakeable silhouette.  There are two 747-upper-deck major versions:

The original-length, short upper deck, used on almost all first- and second-generation 747s (the 747-100, -200, and -SP) and on all 747s built as freighters, has one emergency exit per side, and the hump begins to taper down almost immediately; as a result, the rear end of the hump ends forward of the wings (except on the -SP, whose shorter fuselage brings the hump’s rump rearward over the wing roots, even with the original short hump).  On passenger aircraft, the upper deck was originally used for a first-class lounge, but was later replaced with a few extra rows of passenger seating; on freighters, it contains a few rows of seats for crew rest and carriage of supplementary/supernumerary crew, cargo-handling personnel, and nonrevenue passengers.
The later, stretched upper deck, available as an option on the very last passenger -100s and late-model passenger -200s, and used on all -300s3 and passenger -400s and -8s,4 has two exits per side, and produces a hump that extends well back over the wing roots; it provides a considerable increase in passenger capacity for the 747s so equipped.

As even relatively-new 747s become increasingly uneconomical to operate as passenger aircraft in a world of rising fuel prices and big, efficient, ETOPS-300+-certified twinjets, many long-hump 747s are being converted for freighter service; the conversion process for these aircraft (known as Boeing Converted Freighter, or BCF, 747s) involves stripping out the main-deck seating (and ancillaries like lavatories, galleys, overhead bins, etc.) for cargo rails and tiedowns and cutting main-deck cargo doors into the fuselage.  It does not, however, involve cutting the upper fuselage down to a short-hump configuration (which would be a far more involved piece of surgery than the other BCF modifications), and, as such, leaves the BCF aircraft with an upper deck that is all wrong for its new role; it can’t be used for large, containerised cargo á la the main-deck and lower-lobe5 compartments (the upper-deck fuselage profile doesn’t leave enough space, and there’s no upper-deck cargo door), and, yet, it’s far too large for carrying any reasonable amount of supernumeraries, cargo-handlers, or would-be jumpseaters.
What do operators of long-hump 747-BCFs do with the upper deck?  Do they leave the seats in, on the off chance that they have to transport something that requires a LOT of handlers/escorts/whatnot?  Use it for small bulk cargo (mail, parcels, checked baggage that didn’t make its owners’ flights, etc.)?  Tear out everything but the front few seats and just haul it around empty?

1: ...well, except for these ones, but those are a very special case.
2: The proto-747 lost out to the Lockheed C-5, which has a full-length upper deck used for personnel transportation.
3: The -300 had no dedicated freighter version, although many passenger -300s were eventually converted to carry freight.
4: The -8’s upper deck is slightly longer than that used on earlier long-hump 747s, due to the 747-8’s stretched fuselage.
5: The space (divided by the wing centre box into separate forward and aft compartments) below the main-deck floor, which, in the aircraft’s previous passenger-carrying life, was known simply as the “cargo hold”.

Actually there does not seem to be much of the strategic airlifter proposal left in 747. More direct influence was that freighter use was planned for the 747 itself as a backup if it didn't catch on for passengers or was superseded by supersonic aircraft.

The ones I've been in always had business class/1st class seats and maybe the lounge forward if there was one.  I did a trip to Paris in an Air France '47 and my employer at the time had a business class travel policy for intercontinental travel, so I got to sit in a business class seat in the hump.

Obviously fuel economy is relevant but I wonder how true the sentiment "in a world of rising fuel prices" really is... Oil has been pretty cheap for a while!


text:
Why does adding heat to the subsonic flow increase the speed of the flow?
In Rayleigh flow, adding heat to the subsonic flow increases the speed and adding heat to the supersonic flow decreases the speed.
But what is its physical cause?

First, keep in mind that Rayleigh flow only applies to a constant area duct.  It is a model of flow in an inviscid pipe with constant area.  Second, adding heat does not increase/decrease the speed -- it increases or decreases the Mach number.  This generally works out the same, but the speed of sound is also changing as you add heat in Rayleigh flow.  Here are some nice online notes https://kyleniemeyer.github.io/gas-dynamics-notes/compressible-flows/heat-transfer.html

but adding heat increases velocity as well as mach number at the same time !

Indeed.  Fundamentally, adding heat is adding energy.  The acceleration subsonically isn't very mysterious -- it is the drive towards choked flow supersonically that is the real wild part.  It is worth making sure you understand choked flow in a converging-diverging nozzle (no heat or friction) before moving to Rayleigh or Fanno flows.

I'm not trying to be mysterious or tricky.  @sophit Unfortunately, I don't remember enough of it to give an intuitive explanation.  My gas dynamics classes were more than 25 years ago and I remember it seeming like magic at the time.  I don't really know that I had a true understanding of this one -- it may have been something that I just accepted (and never had the time/opportunity to revisit later).  I remember that choking in a variable area duct came first...

Let's take as an example the brayton cycle, as used in turbojets.
in the combustor section, heat is added to the flow by burning fuel in it at constant pressure. Adding heat to the flow makes it want to expand and occupy more volume. Now since the mass flow rate entering the engine is the same as that leaving the engine (conservation of mass), and since the exhaust occupies more volume than the intake, it is necessary for the exhaust flow to exit the engine going faster than the intake flow.
This in turn means that the momentum of the flow entering the engine is different (less than) the momentum contained in the exhaust flow. A time change in momentum has the units of a force; in this case that force is the thrust of the engine.

Adding heat to a subsonic flow increases the speed of the flow because it increases the temperature of the gas molecules, causing them to move faster and thereby increasing the velocity of the flow. This is governed by the ideal gas law, where an increase in temperature leads to an increase in the kinetic energy of the molecules, resulting in higher velocity.

So why doesn't that also apply to supersonic flow?

"Adding heat to a subsonic flow increases the speed of the flow because it increases the temperature of the gas molecules, increasing the velocity of the flow" That's not correct. Higher kinetic energy only means that each molecules is moving faster but that has nothing to do with the average speed of the whole molecules, average speed which might well be zero. If you heat up you oven, the air inside it does not become supersonic, it just stay there where it is no matter the temperature... "This is governed by the ideal gas law" In that law there is no velocity given...


text:
Do the B777x and similar aircraft use ring laser gyros or solid state MEMS gyros?
In new generation aircraft like B777x , B787 or A350 use the IRS system which is present is it based on (Ring Laser or Fibre Optics) gyros or they used Solid state MEMS like which our present in our smartphones and G1000 avionics?

The A320NEO and B737MAX are not new generation aircraft. They are re-engined last generation aircraft. Their IRS is the same as the A320 and 737NG. The newest generation aircraft are A350, B787 and B777X.

So B777x and  other have same IRS or MEMs

My point was your question doesn’t make a lot of sense. The 737MAX and 320NEO are not “new” aircraft when you talk about the IRS. I believe there are existing questions that address those systems. You should search ASE before asking a question. If you changed your question to ask about the B787 and B777X, it would be clear and understandable. And you could get a useful answer as they use the Honeywell Earth Reference System. That is a redundant hybrid system that uses both RLGs and MEMs. Edit your question and you’ll have a better chance of getting an answer.


text:
Do jetliners carry radar that can detect other aircraft?
If not, why not? For example, if a civilian jetliner is flying at night over the Ocean, or a remote area with no airports/ground radar stations, and another airplane is also heading towards it on a collision course, shouldn't the pilot be able to spot the oncoming airplane on its airborne radar to perform evasive maneuvers? My understanding is that military aircraft have such airborne radar, which I believe is called ACAS, so they need not depend on ATC/ground stations for collision avoidance.

Air Traffic Control is responsible for preventing collisions. In areas where there s no ATC, pilots position reports, traffic advisories, and TCAS do a good job preventing collisions. Airborne Weather Radar actually will pick up another aircraft, but it can’t be relied on for accurate positioning.

Radar that can detect, resolve and track small targets as opposed to large areas of water (raindrops) has to be a lot more powerful and sophisticated, especially in ye olden tymes of the 50s and 60s when weather radar was becoming a thing and the search for a practical collision avoidance system was underway.
When the effort that eventually became TCAS got started in the late 50s following several notable midairs, on-board target detection was one of the early proposals but was rejected by the industry as too expensive and time consuming to develop (where to put the more powerful system with its larger antenna for starters).
Once TCAS was perfected, made possible by the development of Mode C altitude reporting by transponders, you got 95% of the benefit (because only non-Mode C-transponder light planes were still a risk - a very minor factor overall once Mode C was pushed out to GA aircraft) without a crazy expensive addition of on-board target detection and tracking radar PLUS a weather radar.

Cessna Citation I weather radar.

SOCATA TBM 900, weather radar housed in the pod on the left wing
Weather radars are tiny compared to fire control radars. I believe this is because they do not need to collect as much info about their target as fire control radars. Further, they do not need to produce beams as narrow or as fast-moving as FCR's. Keep in mind that raindrops move much slower than aircraft, thus a weather radar does not need to track a flanking (moving sideways relative to the observer) target at 500 knots.

Zaslon AESA radar of the MiG-31, 1.4 meters in diameter.
FCR's consume significant electrical power and they require constant attention from their operator to be  directed to a certain part of the sky and be effectively used.
With TCAS/Mode C transponders onboard most aircraft, there is simply no reason to put an FCR on an airliner or any civilian aircraft for that matter. Most airliners don't even have a large enough radome to put an FCR in.

No, TCAS does not rely on ground based radar stations and it works perfectly well at night over the ocean. Each aircraft broadcasts messages, which include altitude, and listens for messages from other aircraft. If TCAS receives messages from another aircraft nearby that operates at the same altitude, it warns the pilots and advises them to climb or descend.
Unlike radar, TCAS only works if both aircraft are equipped with it.

The question isn't about TCAS even though it has that tag. It simply asks if commercial aircraft have radar that can detect other planes.

@CareyGregory that is correct. Perhaps I should remove that TCAS tag.

TCAS is a form of radar. Also, the last sentence of this answer is incorrect - TCAS does not require both aircraft to be equipped with it. It's sufficient for one of the aircraft to have TCAS and the other to have a Mode C or Mode S transponder.


text:
Are details logged anywhere on a go-around?
On March 1 I was a passenger on an American Airlines flight (AA 2315) landing at Denver International Airport, during the landing the pilot accelerated unexpectedly and started a turn.  He came on the intercom and explained that a Southwest flight had gotten too close, and so they decided to loop around and try the landing again.
Everything was fine, but I'm just so curious - are details about this incident logged anywhere?  I'd love to know more!
EDIT
There is a great post that talks about all the different factors that go into determining whether an aborted landing is considered an "incident" or not.  Unfortunately, I don't know enough about the details to be able to determine whether my particular experience is an incident.  I'm hoping that incidents are publicly posted and I can search there to find out if my landing shows up or not.

Does this answer your question? Is an aborted landing considered an incident for the FAA? - specifically my answer there

Great information!  Unfortunately, as I was in the middle seat on the exit row (so no window) I don't know a lot of the details (like how far off the ground we were).  Is there an online database that I can look at to see if it was reported (which seems unlikely)?

I looked it up on the history replay function of FR24.com. Your flight was co-altitude with a Southwest 737-700 that was turning onto approach to a parallel runway, at less than 1NM on a converging course. In my book, that qualifies a serious incident.

The most annoying part of all stack overflow sites, is the overzealous admins.  My post is NOT the same as these other posts - I'm not asking a general question, I'm asking a specific question about a specific incident.  DeltaLima actually answered the question, at least got started in the right direction (thank you!)

@DeltaLima - I can find my flight on fr24.com, but can't figure out how to find that Southwest 737-700.  How do you look at two flights at once?  And do you remember the flight # of the Southwest plane?  Thanks!

@DeltaLima - maybe serious, maybe not. Runways 16L/R are only 2600' apart so when they run simultaneous parallel arrivals (as they're doing right now) the aircraft will get closer than a mile, at the same altitude. But there are specific procedures that have to be followed, which I'll not go into the details of because I don't have any sort of familiarity with them. It's very possible that this could be a "serious" incident and it's also possible that SWA just drifted a little too close. I don't know which it is, but just saying.

@BettyCrokker Just so you are aware, and think poorly of our actual moderators (they have diamonds after their usernames), the 5 people who voted to close your question as a duplicate were regular users, albeit with high reputations, or high enough to be able to do so. Stack Exchange is a community curated question and answer site. A diamond mod could close it, but they generally leave the "easy stuff" to us community members to do.

@CGCampbell Thanks for the clarification.  I amend my previous post to "The most annoying thing with all stack overflow sites are the users who overzealously close questions" :-)


text:
What is the difference between autopilot and fly-by-wire system?
I observed a lot of people misinterpret these words. In my opinion, autopilot assists the pilot by inputting the commands to the computer whereas fly-by-wire (FBW) system sends and receives signals from control surfaces.

I have heard Airbus's FBW system overrides the pilot commands most of the time. Does this imply that the aircraft is controlled by a FBW system? What about Autopilot?
When aircraft encounters sudden gusts, does autopilot contribute to making the aircraft stable? I know FBW will send signals to the different control surfaces to act accordingly.
Which system controls the aircraft and offers carefree maneuvering? Which system assures the aircraft will remain safe and stable at every point?

In bullet 3, I believe both systems make sure the aircraft remain safe and stable. The autopilot makes landing, takeoff, and flying safe and easy with the help of ILS, autothrottle, etc. and FBW assures that every control surface is actuated in a safe manner which means it is related to attitude control, right? How do both systems work hand in hand?

Is this question about Airbus FBW specifically, or FBW systems in general with the Airbus system you mentioned just being an example?

FBW in general not particular to any company

I think the answers are unnecessarily complex. There's a waterfall of increasingly fine-grained tasks from the strategic goal of a flight, to millisecond-level individual inputs. The point in this hierarchy at which it stops being the responsibility of auto* and starts being considered fly-by-wire is the point at which it is equivalent to (and typically mixed with) pilot input on the controls.

Stability can be aided by autopilot and FBW but for most normal aircraft neither of these systems ensure the aircraft remain safe and stable. Instead the aerodynamic design and CG location of the aircraft is what makes it safe and stable. It is the same mechanism as what makes some paper airplane designs more stable than other paper airplane designs. The exception being aircraft designed to be deliberately unstable such as jet fighters (mostly ONLY jet fighters - nothing else really benefit from not being able to fly without a computer)

The title question itself is problematic, as the two systems are not comparable and therefore can't be properly differentiated. Then, the additional questions would be cause for closure on most SE networks as needing focus. They each warrant full answers.

Agree with @isherwood; OP should appreciate the title of the question is perhaps like asking "what's the difference between a professional chef and an induction stove?"

I'd support keeping it open if the three followup questions were excised. The existing answers are indeed helpful, and they primarily address the first question.

An autopilot can fly the aircraft based on pilot selected modes. The autopilot therefore reduces the pilot's workload by replacing manual inputs via the yoke or sidestick. A fly-by-wire (FBW) system is a type of control system where mechanical connections between the flight controls and the control surfaces are replaced by electrical connections. Usually, there is also a computer involved in between which can adjust or restrict the signals sent to the control surfaces.

To answer your specific questions:

I have heard Airbus's FBW system overrides the pilot commands most of the time.

The Airbus FBW system is active all of the time, but it does not override the pilot input most of the time. It will only override pilot input, if such input would get the aircraft out of its nominal flight envelope. The FBW system therefore provides flight envelope protections. These protections are however not active in every control law (see busdriver's answer).

Does this imply that the aircraft is controlled by a FBW system? What about Autopilot?

The aircraft is controlled by the pilot or autopilot via the FBW system. The pilot or autopilot (A/P computer) send their input electrically to the flight control (F/CTL) computers, which then actuate the control surfaces based on the current control law:

(Airbus A320 FCOM - Flight Controls - Description)
Since both pilot and autopilot inputs are going through the flight control computers, all flight envelope protections are still provided by these computers, even if the autopilot is active.

When aircraft encounters sudden gusts, does autopilot contribute to making the aircraft stable?

All transport category aircraft must be statically and dynamically stable, even if the autopilot is turned off. The FBW system may however assist in achieving the stability (e.g. in yaw damping, see this answer).
If you mean maintaining a constant heading and altitude in gusty conditions, that must be done by the pilot or autopilot.

Which system controls the aircraft and offers carefree maneuvering?

Control is either with the pilot or the autopilot (never both: if the sidestick or rudder pedals are moved beyond some threshold, the autopilot will disconnect). I'm not quite sure what you mean with "carefree maneuvering", but all flight envelope protections and other assists (like e.g. automatically maintaining altitude when banking, see this answer) are provided by the FBW computers.

Which system assures the aircraft will remain safe and stable at every point?

As long as the flight envelope protections are active (depending on control law), the FBW will make sure the aircraft remains safe. In degraded control laws (especially in direct law), the pilot is still responsible for keeping the aircraft safe and stable.
The autopilot does not really care for safety though: if you tell the autopilot to climb at 6000 ft/min at cruise altitude, it will happily attempt to do so, but the flight envelope protections will kick in before the airspeed drops too much and the aircraft stalls.

Fly-by-wire system removes physical link between cockpit controls and flight control surfaces. To do this, there must be some sort of rule how pilot's input is interpreted and converted into control surface movement. This is called a control law.
In Airbuses there are three* different kind of control laws. "Normal law", "alternate law" and "direct law". In normal law side stick inputs are interpreted as G-load demand and roll rate. In direct law there is direct relation between side stick position and control surface deflection. In normal law there is also flight envelope protection, in direct law there is no protections what-so-ever. Still they all are fly-by-wire. I've flown several different airbuses for many years, and not once I've had a malfunction which would have degraded control law from "Normal law" nor not once has fly-by-wire system restricted my manual control input.
The flight envelope protection is there do exactly what the name implies: it will not let you stall or overload the structure. If you stay within the designed envelope you will get the full performance the aircraft has to offer. Even more, if you need the maximum available performance for any reason, like to suddenly avoid a mountain top, you can pull side stick back and let the computer do its magic without fear of over stressing the airframe.
The autopilot is there to control the pre-programmed flight path, ie. maintaining altitude, turning and maintaining to specified heading or tracking the flight plan.
The difference is that for example if sudden gust causes aircraft to climb for example 100ft, the autopilot will return to the preassigned altitude. If you are flying the same flight path in manual control the same gust will leave you in the new altitude and a pilot input is required to return to the desired flight path.
If autopilot fails, the flight can be operated with manual control with all the goodies available. Then again, if critical control computers should fail it might be that besides degraded control law the autopilot is not working either. The autopilot drives the control surfaces through the same computers as the pilot.
*There are more, and mixtures of different modes but this is the basic concept

See Bianfable's answer for more details. Basically, the two systems don't work together at all. Technically both systems (side stick and autopilot) drive their commands through the same computers but never at the same time.
If autopilot is engaged and pilot makes a control input the autopilot will disengage. Also if autopilot is commanded to do something that makes flight envelope protection to kick in (like keeping constant climb of 6000 ft/s) the autopilot will disconnect.

+1 for explaining the control laws and, especially, for This is called a control law.

Do you know what the origin is for the use of the word 'law' here? I may be wrong but aviation seems to be the only field that uses it in this way.

You have already received a number of excellent answers, and I just want to add something very short and to the point, regarding the question you pose in your title:

What is the difference between autopilot and fly-by-wire system?

Autopilot refers to who is in charge of the flight control inputs, Fly-by-wire refers to how flight control inputs translate into control outputs. The autopilot makes its own inputs, FBW translates inputs it gets from someone else (either the pilot or the autopilot) into outputs.
Without FBW, there is typically some form of direct mechanical linkage between the flight control inputs (yoke, pedals, levers) and control outputs (aerodynamic control surfaces, throttles, etc.) With FBW, there is some form of electronic or computer support involved. The extreme case is that there is no direct link at all: a sensor in the yoke measures the angle and sends a command to a motor on the elevator, for example.
FBW does not make control inputs on its own, it only translates control inputs into control outputs.
The autopilot makes its own control inputs. This does not require any FBW at all. There are autopilots which simply pull on the same cables that the pilot does. Even though this is not typically how it works, you can think of it as follows: no matter how simple and mechanical your airplane is, in the worst case, the autopilot could always simply consist of a set of motors that do exactly what you would do – you could even have an autopilot for a hang glider that shifts some weights around on ropes. Or, you know, this.

Excellent for reminding us that there were autopilots long before FBW had been developed. (Think WWII era.)

+1, for finding the short wording: Autopilot determines what must be done, FBW determines the best way to do it.

fIn short
A very simple way to understand the difference between autopilot and fly-by-wire is to compare them to an autonomous car and its anti-skid system.
A more aeronautical way is to look at how a turbine engine is actually controlled: The pilot gives their orders using a throttle, but orders are overridden by the FADEC on the engine, which actually manages the engine controls. Even when the auto-throttle is on and orders come from a computer instead of a pilot, the FADEC still intercepts them. In both cases:

The autonomous car computer and the auto-throttle are autopilot functions.
The anti-skid and FADEC are FBW functions.

They live their lives independently of each other, exactly like the autopilot and the control surfaces FBW computers live independently of each other.

Autopilot is a pilot
Autopilot, or flight computer, is comparable to a substitute for the pilot manual inputs. What does the flight computer could be done by manual inputs. However why would you send manual orders to fly a holding pattern if a computer can do it automatically, managing wind, speed, bank angle limits, etc, itself?
For Airbus the autopilot is the Flight Management Guidance System (FMGS).
Flight-by-wire is a elaborate anti-skid system
FBW is in my opinion a very bad choice for the concept it covers, as it introduces the idea there are wires and electronics between the pilot hand or foot and the control surface actuator.
There are actually wires and electronics, but wires are not a requirement, this could be a mechanical linkage assisted by electronics. And actually in Airbus FBW there are both wires and mechanical linkages.
The point of a FBW system is the orders received, regardless of whether they come from the pilot or the flight computer, are "managed" by a flight control computer. In Airbus avionics, the crew and the FMGS (autopilot) send commands to redundant computers:

2x ELAC (elevator aileron computer) for elevators/THS and ailerons,
3x SEC (spoilers elevator computer) for elevators/THS and spoilers,
2x FAC (flight augmentation computer) for rudder,

Airbus fly-by-wire, Source
Computers send commands to actuators. ELAC, SEC and FAC have nothing to do with the FMGS, the computer used for autopilot functions. FGMS inputs are visible above at ELAC and FAC level.
To simplify: FBW is like an anti-skid system. The brakes on a car may receive a command which is too hard, and known to lead in certain conditions to skidding and a less efficient braking force than a smoother order. The brake computer (anti-skid) will then overrides driver inputs and translates the original order into a more efficient brake command.
The computer is able to react very quickly and to analyze actual braking in order to modulate the command. The role of this anti-skid system is independent of how the order was sent, by a human or a computer.
FBW computers are the anti-skid systems used to control rudder, elevators, ailerons and other control surfaces. They maintain the aircraft in its flight envelope, that is in the domain it is able to fly safely and efficiently, and where the control surfaces are operative (e.g. if the airspeed is too slow on the ailerons, they will be stalled, the flight envelope protection acts to prevent such possibility).
FBW laws
The way FBW work is based on ELAC and FAC being operational and receiving inputs from a lot of sensors, like the ADIRU (air data and inertial reference). If one of these computers fails or lacks inputs, then their roles is redistributed in a degraded way, and if nothing works, then control is returned to the pilot. This progressive "de-automation" is done by switching the "flight laws". Degraded laws are:

Alternate law (two levels: With and without reduced protections),
Direct law (no protection),
Mechanical (mechanical linkage between controls and control surfaces).

Airbus flight control laws, source
See Is a Control Law Degradation in Airbus Planes displayed on the ECAM? for more.
Answers to your questions

Yes most of the time autopilot (Airbus FMGS) is active, and therefore send orders to control surfaces to fly according a pre-determined route. It manages track errors.

Yes the flight control computers (Airbus ELAC, FAC, SEC) always override pilot inputs. This is still true when the FMGS actually pilots the aircraft. (This is comparable to car drivers orders always overridden by the anti-skid, even when a computer drives an autonomous car.)

In case the flight control computers fail partially or totally, some degraded ways to override pilot inputs are used, in order to continue protecting the flight envelope as most as feasible. In last resort some functions are completely returned to the pilots: This is the mechanical linkage step. In that case a pilot is able to manually control rudder and elevators.

Note one axis (pitch, roll, yaw) can be under some law and another under another law. So it is incorrect to say an aircraft is under alternate law in most cases.

FWB is like an anti-skid system. should read FBW ..., I assume.

The question seems based at least partly on the premise that "autopilot" and "fly by wire" are magical electronic computerized systems. While that is true with modern Airbus (and I believe some but not all recent Boeing models), that is not always the case, particularly with smaller airplanes.
Autopilot, in some fashion, has been around for over 100 years. While modern autopilots can take a series of waypoints and other directions and "do it all", even landing the plane, more basic autopilots manage some key metrics (e.g., speed, altitude, bank angle) and adjust engines and control surfaces to keep the plane on track. They can do the basic functions without any digital computers at all using mechanical and hydraulic linkages between controls set by the pilot and control surfaces and engines.
Fly-by-wire really has two main components, in my limited understanding. It uses electronic connections over wires (no, you don't control your elevators via WiFi from a tablet in the cockpit, I hope) in place of mechanical and hydraulic linkages, providing weight savings (which means less fuel and/or more cargo) and some other physical advantages. This doesn't technically need any computers - a simple dimmer switch could (in theory) be set up to control some remote part of an airplane over a couple of wires with no computers (not even a microcontroller) involved.
It also includes (in all current implementations that I know of, but I'm not an expert) a computer in the middle that interprets the inputs to drive the outputs, allowing for envelope protection and other features to be included (as very well explained in other answers). You could have fly-by-wire that is simply a translation of mechanical/hydraulic linkages into electrical connections. But once you are already doing that (which requires new designs and certification), it makes sense to include the computer and all the safety and efficiency features it can provide. That doesn't make as much sense on a small aircraft due to the relatively fixed cost of adding the computer (actually multiple computers for redundancy), but in a small aircraft the mechanical and hydraulic linkages are not as big a deal.
An automobile comparison is cruise control. Cruise control has been around for over 100 years on cars, and even longer on trains, long before there were any digital computers. On the other hand, while most cars are still not fly-by-wire (they have mechanical/hydraulic linkages between the steering wheel and pedals and the engine, wheels and brakes - but with power assist, much as airplanes have had for a long time), cruise control is usually computerized now to at least some degree.

"control your elevators via WiFi from a tablet in the cockpit" this is why script-kiddies aren't allowed to build a plane & fly it without some sort of regulatory oversight!

Also, quite a few cars (more at the high end, ATM) are coming out with "FBW" controls. There is no longer a direct, mechanical connection between the throttle and brake (or even the clutch, for those who remember what that is), and the engine or brake calipers.

@FreeMan Driven (pun intended) by electric cars, where the motor is totally FBW by definition. I think I read somewhere that Teslas, even with regenerative braking, do actually have a mechanical/hydraulic link between brake pedal and brakes, but I don't know for sure - and certainly that will become FBW at some point (if it isn't already).

@manassehkatz-Moving 2 Codidact: The brakes are hydraulic for a couple of reasons.  First, fail safe.  If your electric system fails, the hydraulic brakes still work.  (Same reason power brakes still work if the engine quits.)  Second, regenerative braking is proportional to speed: the slower you're going, the less effective it is.  Without some complicated engineering, it wouldn't work at all when you're stopped.  So if you for instance tried to hold the car on a steep hill with regenerative brakes, it'd slowly slip downhill.

@jamesqf Agree with regard to fail-safe (which AIUI was (maybe still is for some people) the big issue with FBW on airplanes). And I understand with respect to regenerative braking. But there really is no technical reason you couldn't have the pedal be "just a sensor" that feeds into a computer which works out any combination of regenerative and hydraulic braking - i.e., keep hydraulic braking as an output even if it has no physical connection to the input.


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Aircraft data bus and computer system
What is a line replaceable unit (LRU) in an aircraft data bus system?
What kind of components are defined as LRU in a data bus system?

Possible duplicate of which is the difference between Line-replaceable unit(LRU) and Integrated Modular Avionic (IMA)?

Not a duplicate.  The IMA uses a databus, and some databus components may be modules in an IMA cabinet, but it is not the data bus.  The databuses do have stand-alone components.

The components needed for a databus depend on the specific bus standards chosen. The most common of the recent air transport (B787, B777X, A350XWB) aircraft use the ARINC 664 network standard.
These aircraft also use ARINC 653 IMA computing resources hosted in a cabinet. Within the cabinet will be ARINC 664 network switch modules. These connect the IMA modules via a backplane bus and also connect it to external systems via fiber optic or quadrax network cabling.
ARINC 664 is a switched Ethernet type of bus with each A664 end system connected directly to a network switch, so there is no contention for media access at the end system. All network access control is controlled within the switches.
Beyond the IMA switches, the network typically has 4 or 6 remote switches spread along the aircraft. Aircraft systems with A664 end systems can be connected directly to these switches, but many systems use legacy interfaces such ARINC 429, ARINC 825 (CAN), and even analog audio or differential voltage signals.
These other networks can be connected via generic programmable network bridge devices called Remote Data Interface Units (RDIU) or something similar. Specialized interfaces are typically identified as gateways (Audio Gateway unit or A825 Gateway). These have an A664 connection as well as a number of the legacy bus connections. These have internal software that translates the data between the bus types. The key benefit of this architecture is a significant reduction in network cabling.

Down to the smallest unit in a system that can be removed as a piece in the field (the line) is an LRU.  So take a computer unit where the internals can only be serviced in a back shop but the computer itself can be swapped out at the gate.  The computer is the LRU.
If you have a black box with an easy to remove cover, with sub-components mounted on quick change cards so techs and open the box on-aircraft, replace a card, close the box, ops test and go, then the card is also an LRU.
The easiest way to tell is, generally speaking if something is an LRU, there will be a removal/installation procedure in the Aircraft Maintenance Manual.  Below the LRU level, it is covered in a Component Maintenance Manual and would be serviced in a back shop or at a 3rd party after being pulled from the aircraft.


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Identify an USAF transport aircraft in Greenland around 1960
Could anybody help me identify this aircraft?
It was photographed in Greenland around 1960.
Thanks in advance.

Welcome to Aviation.StackExchange!

Satoshi, it appears that you've two accounts (an unregistered one from where you posted the question and a registered one from where you edited the question). Please visit this URL to merge the accounts.

That is the Douglas DC-6 / C-118/R6D Liftmaster.
This specific plane was built in June 1947 with modifications for presidential use. It is well known for flying US President Truman. It was the first presidential aircraft that received a distinctive exterior.
It actively served USAF until 1965 when it was transfered to an USAF museum. It is now on display at USAF museum in Dayton, Ohio, USA.
[Source]

[Image Source]

I'm not convinced OP photo is a C-118. It might be a C-54.  I'm looking at the prop hub and the C-118 had a spinner that covered the prop shank more than I can tell here. I'm not certain; does anyone have more afirmation?

@radarbob, The C-54 had round passenger windows, and only one window behind the rear door. See for example https://www.nationalmuseum.af.mil/Visit/Museum-Exhibits/Fact-Sheets/Display/Article/195813/douglas-vc-54c-sacred-cow/ .


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How is device control software checked for quality?
Many, many reports on the now infamous MCAS suggest a software patch is forthcoming in the next several weeks that will address the confounding single point of failure with the two AOA sensors that probably contributed to at least one 737 MAX crash.
As a software developer (of a much different variety than a Boeing engineer), I am very curious:

What do regulators and the public know about the actual code that flies planes (and sometimes crashes them)?

I've been looking for an hour and can't locate a single patent application that even mentions what language it's written in, though I imagine it's a complex network of machine languages and so forth. I tend to doubt the 737 runs on JavaScript...
More importantly:

Is there any procedure for QA'ing an airline manufacturer's code?
Does someone at the FAA get invited to Boeing's private Github account?

I'm mostly being facetious, but you get the idea. I ask because every day it sounds more and more like a freshman error in code cost 300+ lives.

Is there anywhere I can look for descriptions of this mysterious code and its forthcoming patch?

My understanding (I won't post it as an answer) is C or C++ is most commonly used in aircraft computers (along with some other esoteric languages with odd names).  You have to go with tried and true in aviation, especially if you want to certify something in Transport Category,  so you tend to see technologies that are 10 to 20 years behind the rest of the world.  And even then, things still go off the rails.

This topic is somewhat covered in this podcast and is worth a listen.

There's a bit of a flaw in your question. Avionics and embedded software in general are very different from PC software. First aeronautical engineers come up with a spec ("ICD") or prototype model that behaves in the appropriate way. Then software engineers convert that to certified code, which is tested against the spec and then the whole plane is flight tested. It is important to make this distinction because the majority of issues occur in the spec, not implementation, and that aeronautical engineering and flight testing is outside the domain of software development processes.

Put simply, reviewing code will not find issues without considering the aerodynamic, structural, and human factors of the plane because the systems that run the code are inherently tied into this via the real world. It is hardly a "freshman error" in that somebody forgot a semicolon.

There's kind of two parts to your question: what review and audit of the system the EASA or FAA has, and what public information is there about the code that flies on planes. If you could clarify which is more important I can probably write an answer.

Thanks, all! @CodyP, you're right, I amalgamated two points, but I'm definitely interested in both equally. Would you prefer I post them separately? I have no aspiration to understand a single command of code but REALLY want to see what it looks like, even if it's not code in operation. The review and audit system is of equal importance.

All software running on certified equipment will have to follow RTCA DO-178C. Any code developed is going to be closely guarded as trade secrets, so it's is highly unlikely you will ever see code unless you work for a company actually developing the software. @JohnK is correct in that most safety critical code is going to be developed in in either assembly, C, C++, or Ada.

[it sounds more and more like a freshman error in code cost 300+ lives]. The fix may be in patching the code, but I haven't seen any evidence that the error is in the code. From what I have read, it seems to me that the error is in the design of the aircraft systems design and perhaps the related training. The quickest and cheapest solution is likely to add a safety barrier in form of a software patch. That does not mean that the software had a bug in the first place.

I've worked on some certified software in a different domain (and slightly less strict); the principle would be similar here.
Basically the certifying authority will want to make sure that proper risk analysis was done on the system—both software and hardware—and risks were mitigated to get to certain level of reliability.
The authority does not have programmers to actually review the code. They will be checking the requirements, the test coverage, records that all procedures were followed. All code must be traceable to requirements that caused it to be written, tests that cover it, there must be records it was reviewed, that all the tests passed.  All requirements must have their tests, then there have to be various stress tests, exploratory tests, the test plans have to be reviewed. There will be various static checks too and guidelines to follow. It is a massive amount of paperwork to make sure risks were not missed.
And then the required estimated mean time between failures for critical systems is so insanely low—IIRC for critical systems the mean time between critical failures must be (estimated) at least 10⁹ hours—that there is simply no way to demonstrate it by tests alone. Even the hardware won't have that reliability—it is hundred thousand years. So the system has to be designed so that it can fail safe and fail over to a backup and then the mean time between failure of all the systems during single flight is what gets above the target.
So there is procedure, but the maker does it and the certification authority only verifies that it was followed.
The safety critical code is indeed usually written in C, or sometimes C++ or Ada. However, safety critical, and any other hard real-time, code must be written with use of static memory only. This eliminates large class of problems that C code is normally known for, and the simplicity of C makes it easier to statically check—and the makers of critical software have powerful advanced tools for doing it—, so it is actually quite good fit.
The code still actually tends to be pretty ugly. It is hammered heavily with the tests, but when fixing issues found, preference is to do smaller changes to avoid breaking other parts that already passed the tests, which adds to its ugliness. Nobody thinks its perfect either, but that's not the point. The hardware is not perfect either, so the backups, fail-safes and fail-overs are designed to handle both kinds of failures.
And note that the current Boeing issue is actually a problem with the requirements. The trim system may run away for example if the switch gets stuck, which is always a risk with mechanical switch, so there must be a cut-off switch to disconnect it. And, judging from the public information only, it seems whoever designed this system argued that that cut-off switch is sufficient to handle failure of the new addition too, so the system was not considered critical and fault detection was not included in the requirements for it. But in doing so they apparently underestimated the human factors involved—specific details will be subject to analysis in the investigation and it is what usually takes so long.

Update: The article about the recent B38M crash (ET-302) links to an FAA document FAA and Industry Guide to Product Certification that gives overview of the whole process. It is not particularly about software, but the general process still applies to that.

statically allocated memory is preferable, but systems have been certified with dynamic memory allocation features - it's just more costly to do. I'm assuming you meant C++ rather than C in your 6th paragraph, as that is usually associated with dynamic memory

All software running on certified equipment will have to follow RTCA DO-178C.
This is a fairly complex and expensive process that includes audits specifically with the certification authority (FAA, EASA) throughout the product life cycle, including inspections of requirements, design, implementation, and testing.
Any code developed is going to be closely guarded as trade secrets, so it's is highly unlikely you will ever see actual product code unless you work for a company actually developing the software or are the certification authority auditor(s).
A majority of safety critical code is going to be developed in in either assembly (target specific), C, C++, or Ada.

Is there any procedure for QA'ing an airline manufacturer's code?

DO-178C requires developing a rigourous and complex quality assurance process. This is handled largely through both internal auditors and QA engineers, who are not just the software designers wearing different hats. It's also handled through designated certification representatives who do audits, review certification submission documents, and approve unresolved bug/problem reports. Many companies seem to follow standards such as ISO-9000 or CMMI.
This QA process must include audits as explained in FAA Order 8110.49A, called "stage of involvement" audits. These happen throughout the development lifecycle, not just on the final product, and cover, well, everything from whether your planning is adequate to whether requirements match the code to whether your tests meet standards.

Does someone at the FAA get invited to Boeing's private Github account?

What's actually submitted to the FAA is limited, in part because the FAA's main goal during certification is to ensure the airplane was developed with a high-quality process. The FAA relies a lot on the internal auditors and certification liasons, who have access to the code and requirements. The FAA does see a lot of high-level reports like TSO submittals, a Software Accomplishment Summary, System Safety Analysis, etc.  They also see a list of any open bug reports that actually affect the cockpit.  Finally, the FAA is involved in a limited way in the flight testing of the plane.
The FAA rarely reviews the actual code. As selectstriker2 pointed out in his/her answer, a lot of aircraft code is considered trade secrets, and some considerations like export controls may apply. Even avionics suppliers and airframe developers are reluctant to share too much data with each other to prevent trade secret theft.

I can't locate a single patent application that even mentions what language it's written in

See What programming languages are used for equipment onboard aircraft?. I understand your frustration as I've found it hard to research technology other companies are using myself.
Other points
DO-178 has some spots where it relies on the certifying company to be responsible, despite the fact that financial incentives exist to produce "just good enough" software. For example the FAA assumes the company has good design (including human factors), adequate training, strong investment in QA with limited corner-cutting, and no deliberate misrepresentations.
Most importantly, the FAA aims to ensure the software development process is unlikely to produce uncaught errors, not that the software itself is completely bug-free. Many news reports about the recent 737 Max crash sensationalize this as "delegation" or even "self-auditing" but that's how the FAA has always handled avionics- they can't exhaustively test every feature themselves, and largely rely on the manufacturer to decide the complexities of what is and isn't safe. While the FAA has some recognized design standards like TSO's, these standards are very general. In some ways this makes sense- the FAA can't get caught in a cat-and-mouse game of updating their own design requirements for every new variation or feature in avionics.
Among the other certification requirements that apply to your question:

A safety analysis with calculations or models showing
that issues (say, undetected nose-down below 1000 ft AGL) are not
more likely to occur than there severity requires.  These models are often based on redudancy levels and malfunction probabilities (e.g. both AOA sensors will only fail once per 10^-9 flight hours) (see ARP4761).
Both system and software requirements, with corresponding system and
software tests.  As an analogy, you're making sure the house is not
only built according to blueprints but that the blueprints make sense
for a house. This relates to the point @user71659 brought up about validating the design in context of the plane and not just verifying the code has no errors.
Thorough testing, including testing safety features against sensor failures, testing lines of code to an MCDC coverage level, and structural coverage testing to ensure unintended interactions don't occur
Configuration management, including change review and problem tracking. If you're following DO-178C you can't merge in an unreviewed code change or hide a bug report
Tool qualification to make sure the build environment, automatic tests, etc. are well-designed and stable. See DO-330 for more.

This is a wide topic and there's a variety of certification documents that apply to you question, including DO-178C, ARP4754A, and ARP4761.  Whole books have been written on industry practices so if you wish to understand this question in detail I'd suggest you pick up one (my go-to is Leanna Rierson's "Developing Safety Critical Software").

Good overview, I agree on recommending Leanna's book as well - it's sitting here at my desk and I reference it quite often.


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What factors contribute to the poor off design performance of turbofans?
I am looking to understand why turbofans have a higher specific fuel consumption when ran at lower power settings and what can be done to improve it.
How much does the rpm of the various spools and load factor in? Which carries a greater weight in the efficiency? If the engine were at full speed (like a turboprop) but low load would the efficiency drop much? Perhaps a variable pitch fan is the best solution even as difficult as they are to make?
Would it make sense to use an electric motor as a torque fill or to increase the rpm of the compressors when at lower power settings? I have read about regenerative cycle engines which feed heat back into the combustor but these only seem to add a small amount of efficiency in practice.
Thank you.

I am looking to understand why turbofans have a higher specific fuel consumption when ran at lower power settings and what can be done to improve it.

Actually, a turbofan has a higher efficiency at lower than max (read take off) performance. Why? Engines spend the majority of their life at cruise conditions, so compressors, turbines and fans are designed and optimized to deliver the cruise performance at the best efficiency that is possible according the technology at design time.
The manufacturers achieve this by designing the optimal blade shapes and angles for the condition the most amount of time is spent. Obviously, when you are further away from that optimal point the more likely you encounter non ideal conditions which lead to less efficient flow around the blades and as such this is translated to a higher specific fuel consumption. This is illustrated by the compressor/fan off-design characteristics, also known as compressor/fan map. A compressor/fan map shows the relation between the pressure ratio, efficiency and the corrected (non dimensional) mass flow for various spool speeds. This is shown from the Wiki image shown below:

Note that this is the performance of the isolated compressor, when connected to a turbine, an equilibrium (operating) line is established, ultimately, this operating line should follow the path of the "Peak efficiency line". This compressor map also shows than for a high spool speed (N4) to a low spool speel (N0) the efficiency first increases, to later decrease again. This is caused by the design and the way the flow enters the compressor stages and is exactly why the performance of the compressing turbomachinery has not the same efficiency throughout the whole operating area.
If you would sweep an engine (for real or through a simulation) from take-off power to idle power, the specific fuel consumption would decrease to cruise power to increase again to idle power. This graph, from a simulation of high power setting to a low power setting of an older turbofan engine with a bypass ratio of about 5, illustrates that:

Perhaps a variable pitch fan is the best solution even as difficult as they are to make?

Variable pitch fans are needed when the fan pressure rise becomes low (below about 1.35); these low pressure ratios are seen in very large (geared) bypass designs as on the Rolls Royce Ultrafan design.

Would it make sense to use an electric motor as a torque fill or to increase the rpm of the compressors when at lower power settings?

It would make sense to assist during take-off so that you essentially can use a smaller engine. Take-off power is several times more than cruise power. Also, the powers in gas turbines are so high and the electric motors are very big and heavy (and certainly the batteries) that this can be seen entering in the very far future. This is commonly known as hybrid propulsion.

I have read about regenerative cycle engines which feed heat back into the combustor but these only seem to add a small amount of efficiency in practice.

Indeed, recuperating heat from the exhaust (which is one of the 3 major losses in gas turbine engines) would mean that you have to use less fuel to get to the same combustor exit temperature. This has been investigated in the European Clean Sky 2 project called NEWAC. The problem with recuperating heat is that on one side you have the highest (combustor/high pressure compressor exit) pressure, and low exhaust pressure on the other side, causing mechanical challenges. Also the core flow is at an enormous high speed, so there is little time to exchange heat; this means you need a very big surface area to exchange the heat. The bigger the area, the higher the weight, the higher the weight, the heavier the aircraft, the more drag and thus power are required.


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Is winglet height equivalent to duct height?
Winglets increase effective wingspan by causing vorticies to be generated further away from the wing root in proportion to the height of the winglet (if vertical), likewise with winglets on propellers. Therefore I came to the assumption that a duct wall around the propeller also increases it's effective wingspan in proportion to the height of the duct wall above the propeller. Is this correct?

This would seem to be addressed by this discussion https://aviation.stackexchange.com/questions/27416/are-ducted-fans-more-efficient

@CrossRoads No because that says the duct prevents vorticies which apparently isn't true https://aviation.stackexchange.com/questions/13995/do-ducted-fans-eliminate-induced-drag therefore it must merely reduce the effect of the vorticies, Winglets do it by increasing the effective span so I was assuming that a ducted fan does the same?

Back to your original question: winglets on propellers.  I've never seen that, unless you are talking Q-tip propeller (http://aviationweek.com/bca/remember-q-tip-prop if it opens). I have seen pictures of ducts on twin prop planes and rear engine prop planes.  I wouldn't fly with a duct on my single engine nose prop, wouldadd too much weight and limit forward visiblity too much. I have a certificated plane, the most radical prop available for me is a scimitar shaped blade. As it is, a standard pair of constant speed (pilot controllable pitch) blades is $8 thousand USD, budget busting already.

Here's a site on P-tip propellors http://www.princeaircraft.com/theprop.aspx  I'd have to jump thru a lot of hoops I think to be able to use one of these.

@CrossRoads Yeah Q-tips. I wouldn't either, this duct is for hovering.

Airplanes don't hover.  Fee like we've been on a wild goose chase now.

Have you ever heard of a helicopter?

Nothing in your post indicated helicopter.


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How much lift can be generated by the intake lip of a duct?
How to calculate the extra lift gained by the intake lip of a duct? Aside from improving efficiency by preventing tip vortices, if a duct has a bell-shaped intake, this lip around the edge creates extra lift because the incoming air sticks to and curves around it which lowers it's pressure. This pressure difference between the duct lip surface and the surrounding air is what causes the extra lift.
The concept was used on the US military's Hillier Flying Platform with some success however, I cannot find any information on how to calculate the exact amount of lift generated by the duct lip for a given flow characteristic through the duct.
I made my own model but I have no idea if it's accurate, hence the need for experimental data to check it against.

The concept of getting lift from an engine inlet seems highly dubious. The typical velocity of the air entering the engine is about Mach 0.3 to 0.4.  Which means at aircraft speeds higher that that, the air in the intake is slower than the air underneath (or outside) the intake. Slower air has a higher static pressure. So, it won’t be generating lift. But if you have a link for an intake generating lift, please add it.

@Penguin This is for a craft in hover.

OK, but Bernoulli’s law applies to that as well. And, the “lift” force will act around the entire circumference of the duct, so, even if it does exist, would be expected to be canceled out. Maybe you need an illustration in your question that can explain why / how these factors won’t exist?

That may help, even if for UAV: Effects of Duct Lip Shaping and Various Control Devices on the Hover and Forward Flight Performance of Ducted Fan UAVs